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A council byelection triggered by Labour candidate Willow-Jean Prime winning a seat in Parliament via the party list is expected to take place before Christmas.

Mrs Prime, a second-term Far North District councillor, placed third in the race for the Northland electorate with 7219 votes but her high place on Labour's party list - number 17 - means she has a seat in Parliament.

The byelection - which will be by postal ballot - is expected to cost about $45,000.

Earlier she told the Advocate she intended to inform the council of her intention to resign as early as yesterday if the results for the Northland electorate were clear-cut.

The timing of the byelection is strictly controlled by the Local Electoral Act, which states it has to take place within 89 days of the council's electoral officer being informed of an "extraordinary vacancy", unless that would cause the byelection to fall in the Christmas-New Year period. In that case the byelection will start towards the end of January.

If Mrs Prime resigns as councillor before this Thursday, nominations to replace her will be open from September 27 to October 25.

Voting packs will be posted out from November 29 with the last votes accepted and the result announced on December 21.

If, however, Mrs Prime resigns on or after this Thursday, nominations will be open from November 24 to December 22, with voting from January 26 to February 17.

So far Peter Gill and Jane Johnston have said they intend to contest the byelection. Mr Gill, a motoring writer from Waipapa, was John Carter's sole challenger in the 2016 mayoral election, winning 4485 votes to Mr Carter's 10,889.

Ms Johnston, of Haruru Falls, founded the Kerikeri Ratepayers Association and is the secretary of the Paihia District Residents and Ratepayers Association.

She stood in the general election as the NZ First candidate for Upper Harbour, a seat held by deputy Prime Minister Paula Bennett, and was number 47 on the party list.

The byelection will be conducted by Auckland-based Election Services with the cost determined by the number of voters in the ward.

The Bay of Islands-Whangaroa ward, where Mrs Prime held one of four seats, is the largest in the Far North with about 19,200 electors.